Tuesday, May 9, 2017

As CoCoRaHS observers know, it's often a "feast or famine" situation. Heavy rain occurred the last few days of April and the first days of May from the southern Plains into the Ohio Valley. The reason was a slow-moving upper level system tapping into abundant moisture flowing northward from the Gulf of Mexico.

500 millibar map (~20,000 ft) for 7:00 a.m. CDT May 1, 2017

Over the six-day period from April 27-May 2 periods of heavy rain pounded an area from Oklahoma through the Ozarks of Missouri and Arkansas into the Ohio Valley.

Rainfall exceeded 12 inches in some locations with widespread 5-inch plus amounts. The axis of the heaviest amounts extended over the rolling terrain of the Ozarks in southern Missouri and northern Arkansas. A broad band of 10-inch totals extended through this area producing widespread flooding.

On the outskirts of St. Louis the Meramec River was pushed to a new record-flood level, last set less than two years ago on December 31, 2015. Preliminary data indicate the the river crested at Eureka, MO at 46.11 feet, compared to 46.05 feet in December 2015. In Valley Park, MO the river crested at 43.9 feet, just inches below the record crest of 44.1 feet set on December 31, 2015.

River stage on the Meramec River at Valley Park on May 2 and the forecast stages (dotted line)

Location of Valley Park, MO

I-44 and MO 141 at Valley Park, MO on May 2, 2017Photo credit: KSDK on Facebook

The flooding near St. Louis closed portions of Interstates 44 and 55, at one time cutting off access to crossing the river. The flooding closed numerous roads and in some cases washed out large sections of pavement that will have to be replaced.

Section of I-44 at Hazelgreen.Credit: MO DOT

Missouri road closures from flooding on May 2, 2017

Preliminary data suggest that the month will rank as the wettest April on record, with a statewide average just short of 10 inches of precipitation. That amounts to about one quarter of Missouri’s annual average.

In Arkansas flooding along the Black River and the White River is estimated to have caused in excess of $21 million in damage, and losses to agriculture, where some farmland was under four to eight feet of water, are estimated at $65 million. 36 Arkansas counties were declared disaster areas by Governor Asa Hutchinson

The upper level trough that brought the rain to the central U.S. then turned its attention to Canada as it lifted northeast and deepened.

500 millibar map for 7:00 a.m. CDT on May 6, 2017

Several days of heavy rain in southern Ontario, Quebec, and the Maritimes resulted in widespread flooding there. A number of cities, including Montreal, declared a state of emergency due to the flooding. One of the hardest hit areas was Gatineau, Quebec near the border of Ontario. Government offices were closed early this week with numerous roads flooded,.

The affects of the rain and flooding will continue for some time in the Mississippi and Ohio River Valleys. Flood crests are moving down the smaller rivers into the Mississippi and Ohio Rivers.